1. Field of the Invention
This invention relates to a liquid coating composition for use in forming an antireflective film, as well as a photoresist material using said antireflective film. More specifically, the invention relates to a liquid coating composition for use in forming an antireflective film which is capable of sufficiently reducing the multiple interference of light occurred in a photoresist layer during patterning by photolithography, and thereby keeping the dimensional precision of a photoresist pattern. The invention also relates to a photoresist material consisting of a photoresist layer and said antireflective film formed thereon using said liquid coating composition. The invention is advantageously applied to the photoresist layer of chemically amplified type.
2. Description of Related Art
Photolithography as employed in the fabrication of semiconductor devices comprises the steps of providing a photoresist layer on a substrate such as a silicon wafer, selectively exposing it to light with an actinic radiation such as UV light, far UV light, excimer laser, X-rays or electron beams, and performing development to form a photoresist pattern on the substrate. Photoresists are of two types. A negative photoresist is such that the unexposed area is dissolved away during development, while a positive photoresist has the exposed area dissolved away. Either type of photoresists are selectively used as appropriate for a specific object.
With the recent improvement in the degree of integration of semiconductor devices, R & D efforts are being directed to production equipment adapted for microelectronic fabrication technology. Under the circumstances, the use of exposing equipment that operates on short-wavelength actinic radiations such as g-line, i-line or excimer laser is increasing because of its compatibility with microelectronic fabrication technology.
In the formation of photoresist patterns by photolithography, incident light is known to undergo multiple interference in the photoresist layer and that causes variations in the linewidth of photoresist patterns with varying thicknesses of the photoresist layer. The cause of this multiple interference effect of light is as follows: exposing to light falling at a short wavelength on the photoresist layer interferes with the reflected light from the underlying substrate so that the absorption of light energy varies with the thickness of the photoresist layer. The multiple interference occurs a phenomenon known as "standing-wave effect" which gives a wavy cross-section to the photoresist layer, resulting in thickness variation. Variations in the thickness of the photoresist layer will adversely affect the linewidth of the developed photoresist pattern and eventually degrade its dimensional precision. When forming a fine pattern on a substrate having steps, this phenomenon presents an unavoidable serious problem since the thickness of the photoresist layer inevitably differs at different heights of each step. Therefore, it is desired to develop a technology that eliminates the multiple interference effect of light to insure that the dimensional precision of fine patterns will not be degraded even if they are formed on a substrate having steps.
It has heretofore been proposed that the multiple interference effect of light be lessened by forming an antireflective (anti-interference) film onto a surface of a substrate (see U.S. Pat. No. 4,910,122), or by first forming a photoresist layer onto a substrate and then forming a film of a water-soluble resin, such as polysiloxane or polyvinyl alcohol, as an antireflective film over the photoresist layer (see Japanese Patent Publication (Kokoku) No. 55323/1992, Japanese Patent Public Disclosure (Kokai) No. 222409/1991, etc.). The first approach is effective to some extent in lessening the multiple interference effect of light, however, if light having the same wavelength as exposing light is used in mask alignment, the antireflective film will attenuate the alignment detection signal, making it difficult to achieve high alignment accuracy. Additionally, the photoresist has to be transferred patternwise and precisely onto the antireflective film, which must thereafter be etched or otherwise stripped away without affecting the device being fabricated. Since this increases unavoidably the number of process steps involved, the first approach which comprises forming the antireflective film onto a surface of the substrate is not applicable to all cases of substrate processing. On the other hand, the second approach which comprises forming an antireflective film onto a photoresist layer over the substrate is practical since it does not require an intricate process, however, it is not satisfactory in their anti-interference action. As the slightest interference effect can influence significantly the dimensional precision of photoresist patterns if they are very fine and, therefore, antireflective films available today are incapable of meeting the recent demand for smaller feature sizes in the fabrication of semiconductor devices. Under the circumstances, there is a strong need to develop a more effective antireflective film.
In addition, the latest integrated circuits need an extremely fine pattern having a width of about 0.2-0.3 .mu.m or even less. In line with such technical advancement, the conventional photoresist composition for g-line or i-line is being replaced by that of chemically amplified type for deep UV having a wavelength of about 248 nm or less. Requirements for antireflective films are shifting accordingly.
The chemically amplified photoresist composition is highly transparent for high resolution, and therefore, is highly vulnerable to the standing-wave effect. For these reasons, roles of antireflective films overlaid on the chemically amplified photoresist composition have become more important than ever.
From the principle of reflection, the standing-wave effect may be reduced if the refractive index (n) of the antireflective film is equal to a square root of the refractive index (n') of the photoresist layer for exposure light. The antireflective film for the conventional photoresist composition for g-line or i-line should optimally have a refractive index (n) of about 1.29, whereas the antireflective film for the highly transparent chemically amplified photoresist composition for deep UV should optimally have a refractive index (n) of about 1.34-1.36. Therefore, for the chemically amplified photoresist composition for deep UV, it is desired to use an antireflective film having an optimal refractive index corresponding to said composition as described above.
The antireflective film should meet not only the requirement for said refractive index but also the requirement for film quality, that is, an adequate degree of hardness. If the film is excessively soft, it will deform or drip when a silicon wafer is transferred, with a photoresist material thereon of a photoresist layer overlaid with said excessively soft antireflective film, and whereby contaminating the nearby placed apparatus. Further, the antireflective film needs to be removed easily. However, if it is excessively soft, it may not be removed completely from the underlying photoresist layer, which hampers the subsequent development of the photoresist layer.
Conventional photoresist materials of a dual structure comprising a photoresist layer overlaid with an antireflective film have been proposed in Japanese Patent Public Disclosure (Kokai) Nos. 62520/1987, 241332/1993, 110210/1994 and 74700/1993, etc.
Kokai No. 62520/1987 teaches a photoresist material with an antireflective film formed from a perfluoroalkyl compound of perfluoroalkyl polyether, perfluoroalkylamine, or a mixture thereof. However, the said antireflective film has a refractive index of about 1.30, and therefore, it is suitable for a photoresist composition which responds to g-line or i-line. The antireflective film does not have a refractive index of about 1.34-1.36 being required in using a chemically amplified photoresist composition that responds to deep UV. Therefore, the antireflective film in Kokai No. 62520/1987 does not reduce the standing-wave effect when used in combination with the chemically amplified photoresist. It cannot provide a fine pattern required of the latest integrated circuits.
Kokai No. 241332/1993 teaches a photoresist material with an antireflective film formed from a solvent solution wherein fluorine-base resins dissolved. As fluorine-base resins, a cyclic perfluoroalkyl polyether and a chain perfluoroalkyl polyether are exemplified. However, for forming the antireflective film, those two types of perfluoroalkyl polyethers are independently used, and that they are not used in admixed with each other. The antireflective film is being able to adapt to a photoresist of chemically amplified type, however, it could not achieved a sufficiently satisfied advantageous effects in film quality and in film removability.
Kokai No. 110210/1994 teaches a photoresist material with an antireflective film of a gas-impermeable polymer film formed from a fluorine-containing polymer. The gas-impermeable polymer film protects an underlying photoresist layer of chemically amplified type from its atmosphere, by which gives a photoresist pattern in good shape. On the other hand, the antireflective film of the present invention keeps a photoresist pattern in good shape by means of suppressing the multiple interference effects in a photoresist layer. The technical concepts of the present invention differ from those of the invention of Kokai No. 110210/1994. Furthermore, it discloses examples employing a cyclic perfluoroalkyl compound as the fluorine-containing polymer, but it does not teach a chain perfluoroalkyl compound.
Kokai No. 74700/1993 teaches a photoresist material with an antireflective film formed from a solvent solution of a resin obtained by polymerization of a perfluoroalkyl compound having unsaturated bonds. The said antireflective film is being able to adapt to a photoresist of chemically amplified photoresists, however, it is not satisfactory in film quality and film removability, and therefore it is not suitable for fine patterning.
The present inventors conducted intensive studies with a view to solving these problems of the prior art and found that their objective could be attained by forming an antireflective film using a liquid coating composition comprising a mixture of a cyclic perfluoroalkyl polyether and a chain perfluoroalkyl polyether in a specific ratio by weight dissolved in a fluorocarbon organic solvent. The present invention was completed on the basis of this findings.